1. Field of the Invention
The present invention relates generally to matched filtering devices and more specifically to surface-acoustic-wave convolvers.
2. Description of the Prior Art
In signal-processing technology, it is sometimes necessary to filter a wideband noise from a received wideband electrical signal to recover a desired signal that is in a coded information format. A wideband signal can be defined, very generally, to be a signal which exists over a frequency bandwidth that is greater than the minimum bandwidth required to convey the information of interest. In some cases, information may be spread over a relatively wide bandwidth, such that the energy of the information signal is less than the energy of unwanted noise coexisting in the frequency band containing the signal. In signal-processing technology, it may be necessary, for example, to filter wideband noise from a wideband electrical signal in order to retrieve a relatively small-amplitude, wideband information signal that is underlying the wideband noise signals. In the past, wideband surface-acoustic-wave convolvers have been used to filter wideband noise from the desired signal to be monitored. In these devices, a reference signal, which is a replica of the wideband information code signal but inverted from front to back with respect to a predetermined time period, is generated as a surface acoustic wave by an input surface-acoustic-wave transducer. Similarly, the received signal, which contains the desired wideband information signal and the wideband noise signal, is generated as a surface acoustic wave by another input surface-acoustic-wave transducer. Each of these waves travel toward one another and convolve with one another on a convolver plate between the two transducers. The convolution of the two surface acoustic waves is detected by a convolver output transducer located on the convolver plate between the two input transducers. The convolution of the two waves produces matched filtering of the reference signal with the received signal to reduce the wideband noise in the received signal with respect to the desired information signal.
One problem with designing wideband surface-acoustic-wave devices is the difficulty of making a wideband convolver which is linear over the entire frequency range of a wideband signal. Another problem with wideband convolvers is that the output from the matched filtering convolution process may be useless if a narrowband noise signal having a relatively large amplitude is present in the received signal.
As a solution to the first of the above-mentioned problems, a type of wideband convolver has recently been developed which is made up of a plurality of narrowband surface-acoustic-wave devices in which each narrowband device is designed to operate over succeeding portions of the entire wideband bandwidth. Each narrowband convolver may, for example, operate in a bandwidth of 8 MHz. Therefore, to cover a wideband signal ranging from 60 MHz to 124 MHz, it may be desirable to use eight narrowband surface-acoustic-wave devices. It should be noted that with this type of configuration, it is not necessary that each channel operate over an equal bandwidth range. In such a device, a plurality of narrowband signal-processing devices may be placed in parallel. The output from all of the surface-acoustic-wave devices may then be coherently recombined to provide a processed wideband output signal. Thus, it may be possible to effectively process a wideband signal with a plurality of narrowband surface-acoustic-wave devices.
However, as alluded to above, another problem with wideband convolvers is that the signal-processing capability of a wideband convolver may be rendered useless by a narrowband noise signal having a relatively large amplitude. It is to this problem that the present invention is directed.